How to implement sizeToFit if it depends on layoutSubviews? - ios

Question

In the documentation of layoutSubviews, Apple says:
You should not call this method directly.
I'm trying to implement sizeToFit. I want it to put a tight bounding box on all of the subviews. I have to layout the subviews before determining such a bounding box. That means I must call layoutSubviews, which Apple frowns upon. How would I solve this dilemma without violating Apple's rules?
- (void)layoutSubviews
{
[super layoutSubviews];
self.view0.frame = something;
self.view1.frame = somethingElse;
}
- (void)sizeToFit
{
[self layoutSubviews];
self.frame = CGRectMake(
self.frame.origin.x,
self.frame.origin.y,
MAX(
self.view0.frame.origin.x + self.view0.frame.size.width,
self.view1.frame.origin.x + self.view1.frame.size.width
),
MAX(
self.view0.frame.origin.y + self.view0.frame.size.height,
self.view1.frame.origin.y + self.view1.frame.size.height
)
);
}

Answer 1

One should not override -sizeToFit. Instead override -sizeThatFits: which is internally called by -sizeToFit with the view's current bounds size.
You should not override this method. If you want to change the default sizing information for your view, override the sizeThatFits: instead. That method performs any needed calculations and returns them to this method, which then makes the change. – UIView Class Reference
Also not that even if you would override -sizeToFit, there is most likely no reason to perform layout immediately. You only size the view, i.e. set its bounds size. This triggers a call to -setNeedsLayout, marking the view as needing layout. But unless you want to animate the view, the new layout does not have to be applied right away.
The point of this delayed update pattern is that it saves a lot of time if you perform multiple consecutive updates, since the actual update is only performed once.
I typically do this. It works like a charm.
#pragma mark - Layout & Sizing
- (void)layoutSubviews
{
[self calculateHeightForWidth:self.bounds.size.width applyLayout:YES];
}
- (CGSize)sizeThatFits:(CGSize)size
{
CGFloat const width = size.width;
CGFloat const height = [self calculateHeightForWidth:width applyLayout:NO];
return CGSizeMake(width, height);
}
- (CGFloat)calculateHeightForWidth:(CGFloat)width applyLayout:(BOOL)apply
{
CGRect const topViewFrame = ({
CGRect frame = CGRectZero;
...
frame;
});
CGRect const bottomViewFrame = ({
CGRect frame = CGRectZero;
...
frame;
});
if (apply) {
self.topView.frame = topViewFrame;
self.bottomView.frame = bottomViewFrame;
}
return CGRectGetMaxY(bottomViewFrame);
}
Note that the sample code is for a view that can be displayed at any width and the container would ask for the preferred height for a certain width.
One can easily adjust the code for other layout styles though.

Answer 2

I think it's very rare that you will need to use frame when using Auto Layout. Considering you want to solve the problem without breaking Apple rules, I would suggest using the Auto Layout way:
Setup constraints to determine the size of the container view.
Basically the constraints are setup in a way so that the container's width and height can be determined by the auto layout system.
For example, You can setup the constraints like:
Note that view0 and view1 must set both width and height constraints.
When you instantiate the view somewhere in your project, you don't need to setup the width and height constraint anymore. Auto layout will guess its size (called intrinsicContentSize) by the constraints you previously setup.

Answer 3

If you want to force layoutSubviews to happen, but don't want to call it directly, set the needsLayout flag, and then ask it to layout.
[self setNeedsLayout];
[self layoutIfNeeded];